Mass spectrometry is a well-established method of analyzing for the presence and concentration (or amount) of a wide variety of chemical constituents with high sensitivity. Since mass spectrometric analysis includes detection or quantification of various ions having varying mass-to-charge ratios, it is necessary to ionize the molecules of chemical constituents of samples of interest. Heated electrospray ionization (HESI) and atmospheric pressure chemical ionization (APCI) are two common ionization techniques that may be employed to ionize chemical constituents of samples provided in liquid form. These two techniques are somewhat similar in the sense that both require nebulization of a liquid sample spray within a flow of heated gas. However, some fundamental differences exist between the two techniques. The HESI source sprays a nebulized liquid spray where the tip of the sprayer (e.g., a nozzle such as of a capillary tube) has or provides an electrical potential that transfers charge to the droplets. These droplets are then dried by a heated flow of gas before being injected into the mass spectrometer. Although the APCI source also emits a spray of nebulized liquid, the tip of the sprayer does not carry an electrical charge and, in fact, is often grounded. The neutral droplets so produced are dried by a heated flow of gas and then are ionized by way of a corona discharge needle placed between the sprayer and the mass spectrometer.
A HESI sprayer is long enough so that the tip sits outside of the heater region so that the drying gas is heated but the liquid flow is not directly heated. Conversely, an APCI sprayer is shorter so that it sits within the heater region so that the liquid droplet flow is directly heated.
The two above-described ionization techniques are, to some extent, complementary because certain classes of compounds that ionize well in HESI (or ESI) mode often do not ionize well in APCI mode, and vice versa. In some high throughput screening applications, where the amount of sample available is limited and where time is critical, it is desirable to limit the amount of time required in order to identify all the components in the sample. Therefore, it is desirable to be able to switch between the two aforementioned ionization modes with a minimum of time and inconvenience.